Pistons for internal combustion engines are usually produced by casting or forging processes. Production by casting has the advantage that it allows pistons of complex shape and low weight to be produced. However, what has to be accepted at the same time is that the production involves considerable cost and complication. This is particularly true when a steel material is to be used as the material for producing pistons able to withstand especially high stresses.
Depending on their size and intended purpose, forged steel pistons may both be of a one-piece form and may also be composed of two or more parts. In the case of multi-piece pistons assembled from two or more parts, the individual parts are usually connected together, by suitable joining processes, by friction, bonding, or positive fit in such a way that they will withstand the forces acting on them in practical use. What is suitable for this purpose is for example welding or screwing together of the separate parts of the piston.
An example of a multi-piece piston for an internal combustion engine is known from DE 102 44 513 A1. This piston has, on the one hand, a head part which is forged from steel and integrally formed in which are formed dishing for the combustion chamber, an annular wall and a cooling passage in the form of a groove. On the other hand the piston has a skirt part which carries the head part of the piston and in which are formed bosses to receive a piston pin which connects the piston to the connecting rod. To produce this piston, the head part and skirt part of the piston are pre-shaped by forging in separate operations and are then machined by stock-removing machining to finish them. The finish machining of the head part of the piston also includes in this case the stock-removing machining of the portions of wall adjoining the cooling passage, by means of which portions of wall a joint is then made to the skirt part of the piston by physical union by welding or brazing.
It is true that multi-piece construction of this kind allows the piston which is formed from the two parts to be made of a complex shape. However, apart from the problems relating to load-bearing capacity which arise from its multi-piece nature, the cost and complication its production involves are considerable.
Disadvantages of the production of one-piece pistons are the high weight of the blank for the piston, as a result of which processing and handling equipment of particularly large dimensions is required, and the expense involved in the mechanical post-processing which is inevitably required in present-day practice. Despite the advantages that one-piece pistons have, as far as their load-bearing capacity is concerned, the disadvantages mentioned mean that when production is conventional one-piece pistons can only be produced at increased production costs.
One possible way of connecting together, by forging, a piston formed from two previously manufactured parts is known from JP 03-267552 A. In this piece of prior art, a piston-skirt blank whose basic shape is that of a cylinder is produced by sintering a metal powder. A projection which is of a circular disc-like shape is produced on the end-face of the piston-skirt blank when this is done.
In addition to the skirt part of the piston, what is also produced by the known method is a head part of the piston which is likewise of a disc-like basic shape. The diameter of the skirt part of the piston corresponds to the diameter of the head part of the piston in this case. Formed in the end-face of the head part of the piston is a recess whose opening is so defined by an encircling portion which projects into the recess that an undercut is formed between the said portion and the floor area of the recess. To allow the skirt part and head part of the piston to be joined together, the head part of the piston is first placed in a die whose inside diameter corresponds to the outside diameter of the skirt part and head part of the piston. The recess in the head part of the piston faces towards the opening of the die while the said head part of the piston is supported at its other end-face by means of a punch. The piston-skirt blank is then introduced into the die until its projection is seated in the recess in the head part of the piston. The head part of the piston then has a forging force applied to it by means of a shaping punch, which force causes the material of the skirt part of the piston to flow into the recess in the head part of the piston and to fill the undercut which is formed in the latter. The skirt part of the piston is given its cup-like final configuration at the same time.
The piston which is produced by the method from JP 03-267552 A is of an outside shape which is, in essence, completely cylindrical. Formed in the circumferential surface of the skirt part of the piston in this case, closely adjacent to the head part of the piston which is carried by the skirt part of the piston, are grooves for piston rings. Neither the skirt part of the piston nor the head part of the piston have, in this case, any additional configurational features which would make them suitable for a modern-day internal combustion engine. In particular, the known piston does not have any special shaping of the head part of the piston of the kind which is nowadays required if optimum use is to be made of the energy from the fuel which is burnt in the given internal combustion engine. It is also found that simple designs of piston of the kind described in JP 03-267552 A are not equal to the thermal demands which arise in modern-day internal combustion engines.
Comparable possible ways of producing pistons from two parts by means of a positive fit between the parts produced by forging are known from DE 725 761 C, JP 54-021945 A, GB 2 080 485 A or U.S. Pat. No. 3,075,817 A1. However, what all these pieces of prior art have in common is that the pistons which are assembled in a known manner from two parts are each of a simple shape which no longer meets the modern-day demands that are made of pistons for internal combustion engines.